After Updating to MacOs27, none of my APFS external drives mount anymore

To ensure visibility of connected external drives, go to Finder menu > Settings > General tab, tick the box to show External Disks. Change the same setting on the Sidebar tab, if you like.

In Disk Utility select View menu > Show all devices. Does a drive show up there, and if so, can you mount it (button in the top toolbar)?

Disk Utility User Guide for Mac - Apple Support

As you can see from the below, external drives (mine are all GUID/APFS) mount fine on Golden Gate.

If you have 3rd party ‘protection’ software installed, that may be the problem. For example, ESET has been reported to prevent external drives and memory cards from mounting. Such 3rd party system ‘helpers’, ‘cleaners’ and ‘protectors’ do more harm than good. In fact, such products are completely unnecessary for macOS and the only ‘good’ (if you want to call it that) they do is to put money in the developers’ pockets. For the user, they offer only harm…and if that is limited to harm to your bank account, then count yourself lucky.

There were a few prior posts on this forum from users who couldn’t access their external drive due to the drive manufacturer’s software that they used to encrypt the drive. Lacie/Seagate, Samsung, SanDisk, etc., all provide 'drive management' software on their drives and using such 3rd party software can cause issues when macOS is updated and the 3rd party software becomes incompatible. Not sure if that's the issue here, but FYI the best practice with a new external drive (SSD/HDD, not applicable to NAS drives) is to use Disk Utility to complete erase it, format it as GUID/APFS and apply encryption if desired.

Final note: if nothing at all is showing on your Desktop, go to System Settings > Desktop & Dock and make sure the box to Show Items On Desktop is checked.

Odd. Have you rebooted your Mac? If not, that's the first step. If that does not help, then see if the problem is present when you boot in safe mode. That process performs some system cleanup and clears caches, and also disables 3rd party extensions that may cause problems.

Start up your Mac in safe mode - Apple Support

If that does not correct the issue, test in a new user account. A newly-created user account will have all settings at default, and if the issue does not occur in that new account then it provides guidance on what may need to be changed to correct the issue.

Add a user or group on Mac - Apple Support

Once the testing is complete, the new account can easily be deleted.

Delete a user or group on Mac – Apple Support

Those steps will help to isolate the problem.

Ok, that is odd. The enclosure is a thunderbolt capable enclosure. If I connect that into an USB-C hub, effectively forcing it to fall back, it mounts.

It seems that there is at least an issue with the enclosure and Thunderbolt support with MacOs 27.

And even odder - after connecting it to the hub, it now will connect also directly. I will check if I can coax my other drives in a similar fashion.

No security software installed. Also no error msg regarrding sys extension or whatnot.

The drives show greyed out in the disk utility, I can use the info button - but I cannot mount (activate) them. The buttons seems to do nothing. See screenshot.

There also seems another user - also with a MacMini having the same problem. Regarding the APFS part, I could mount a drive with APFS on legacy USB - so it does not seem to be related with the FS. Maybe rather an issue with the external drive chipset in the enclosure? ALl my drives are always freshly partinioned and reformatted due to not trusting any preformatting.

Odd, indeed. In the screenshot that I posted with two of my SSDs connected, both are USB-C SSDs (Samsung T9 and T7) but one was connected directly and the other was connected though a Stone Pro TB3 dock.

Regardless, glad that it seems you are finding a solution.